The Stratigraphy and Selected Properties of Loess in Wairarapa, New Zealand

Abstract

Wairarapa Valley in southern North Island, New Zealand contains a varied and interesting Late Quaternary terrestrial sedimentary record, due to its position in the active tectonic belt that strikes northeast through New Zealand. Active and rapid tectonism are important in producing physiographic features and sedimentary deposits. The valley now has a humid temperate climate with highest rainfall (1200-1600mm) in the north, lowest (800mm) in the middle and intermediate values at the southern coast. The Late Quaternary history is interpreted and compared to other parts of New Zealand and with global Quaternary events. Three sets of Last Glacial fluvial aggradation surfaces are established and each is related chronologically to a loess unit derived from it. Upwards of three loess units with underlying cover beds are also mapped and described from four uplifted Last Interglacial marine benches. The aggradation surfaces and loesses are correlated by lithology and chronology. The Kawakawa Tephra (20,000 years B.P., radiocarbon date), a rhyolitic ash erupted from the central North Island, is a valuable marker bed, both in Wairarapa Valley (where 100mm thick) and other districts. The tephra ensures a North Island correlation of loess and aggradation deposits. The three sets of aggradation surfaces and the three loess units derived from them in order of increasing age are - Ohakea, Rata and Porewa. They formed during cold climate phases when accelerated erosion particularly in the devegetated axial ranges, led to construction of extensive aggradational fans by powerful rivers. Loess derived from the fans and floodplains was deposited episodically on older surfaces. Loess was also locally derived from lacustrine, estuarine and continental shelf deposits exposed by intermittent eustatic falls of sea level. During the warmer periods, the rivers degraded, less loess was produced, and soils formed in the loess. The complex history of deposition and weathering of these deposits is investigated at four selected sites considered to record net accumulation of loess. This record is revealed by detailed study of vertical changes in physical, chemical and engineering properties. Engineering properties of bulk density, Atterberg Limits and permeability are shown to be controlled primarily by water content and thus also relate to grain size and morphology variations within the loess. Grain size, mineralogy and XRF mineralogy record the derivation of the parent material from greywacke-suite rocks, the particle production and subsequent loess weathering. The weathering sequence of Wairarapa Valley loess is a dynamic and reversible pathway linking mica, chlorite and feldspars with 7Å clays, vermiculite and smectite. Volcanic glass is seen to weather either to illite, halloysite or allophane. Diagenesis producing illite within the more deeply buried, permanently moist Porewa loess, is revealed through mineralogy and cation chemistry.